This invention relates to a food, and more particularly to a food having shelf stability while retaining a high moisture content.
Food storage and stability of the stored product are well known problems in the food art. Generally speaking, a high moisture content food is the most palatable, but the least stable. On the other hand, a low moisture content food is the most stable but the least palatable. This distinction applies to foods in general. A discussion directed to pet food, but applicable to all food amplifies to these problems.
Within the class of foods known as pet foods there are three basic subdivisions--dry pet food, semi-moist pet food, and moist pet food. The moisture is determined by considering both the water present in the ingredients of the product and the additional water combined with the various ingredients that make up the final product. In general, the dry pet food--due to its low moisture content which is usually less than about 15 percent--tends to exhibit the greatest microbiological stability and requires no special handling or packaging upon distribution.
At the opposite end of the spectrum is the moist pet food having a moisture content in excess of about 70 percent. Due to the high moisture content of the moist pet food, microbiological stability is a major problem. The problem in a moist pet food is overcome only by heat sterilizing the moist pet food and packaging the moist pet food in a hermetically sealed package. Refrigeration is required after the package of moist pet food is opened to preserve the leftover pet food for even a short time.
Semi-moist pet food has a moisture content generally ranging from about 15 percent to about 50 percent. Since semi-moist pet foods are higher in moisture content than dry and lower in moisture than canned, special techniques are required to render it microbiologically stable. This moisture content in combination with special techniques may produce a microbiologically stable food which does not require refrigeration. At the same time, the higher moisture content of the semi-moist pet food provides an increase in palatability when compared to the dry pet food. The semi-moist pet food is generally rendered microbiologically stable by using appropriate combinations of appropriate solutes. In this fashion, shelf stability without heat sterilization, hermetically sealing or refrigeration is achieved while providing a food having increased palatability relative to dry pet food.
Microbiological stability is a term encompassing a number of features with regard to the storage capabilities of food. This generic term includes three specific stability problems which are common to foods. Firstly, stability against bacteria growth is classified under microbiological stability. Secondly, stability against mold growth is classified under microbiological stability. Thirdly, stability against yeast growth is also classified under microbiological stability. These problems are the problems which must be overcome to achieve semi-moist shelf stability.
When considering palatability, the moist pet foods are the most palatable and the dry pet foods are least palatable. The semi-moist pet foods fall somewhere in between the moist and dry pet foods with respect to their palatability. Thus semi-moist pet food has both advantages and disadvantages.
Another advantage of increasing the moisture content is the improvement in the texture. The increase in moisture results in a softer, more realistic, meat-like (and therefore more desirable) texture for the food. Yet achieving this moisture as above discussed in some cases destroys the desired shelf stability. Thus, it is necessary to balance shelf stability against desired texture and achieve appropriate conditions. In other words, it is necessary to sacrifice this desired texture in order to achieve the desired stability.
As shown by the prior art, especially critical for achieving microbiological stability of a semi-moist food are water activities below about 0.90, because when the water activity exceeds 0.90, as taught by the prior art, a semi-moist product may not be stable. Accordingly, the semi-moist food range has generally been restricted to moisture content below 50 percent and water activity below 0.90.
The microbiological stabilization of a semi-moist food by water activity is usually achieved by the addition of osmotic pressure lowering agents, which in turn maintain the water activity below 0.90 preferably below 0.85. Typically osmotic pressure lowering agents are compounds such as sugar, glycerol, or salts. It is generally recognized that below a level of 0.90 growth of bacteria will be inhibited, whereas yeast and mold are not inhibited until much lower levels are reached. This feature is clearly discussed in N. Potter, "Intermediate Moisture Foods; Principles & Technology" Food Product Development Volume 4, No. 7, November 1970. So a problem exists at water activities in the range of 0.91-0.95, due to possible growth of many different microorganisms.
The water activity (A.sub.w) of food and particularly semi-moist food can be measured by several different methods, such as manometric technique, electric hygrometer and the microcrystalline cellulose method. Of the three methods, the microcrystalline cellulose method has been found comparable to the hygrometer method and superior to the manometer method at water activities in the range covered by this application. It is more useful or economical than the hygrometer or manometric device in that it does not involve the expense of special instrumentation. This method is employed to measure the water activity examples given in this application and is based on the equilibrium moisture absorption of microcrystalline cellulose at a given temperature. This microcrystalline-cellulose method is well-discussed in the art as evidenced by P. T. Vos & T. P. Labuza, "Technique for Measurement of Water Activity in the High A.sub.w Range", Journal of Agricultural and Food Chemistry Vol. 22, No. 2, March/April 1974, pp 326, 327.
In view of the fact that palatability generally increases with moisture, it is extremely desirable to increase the moisture content of a food. It is even more desirable to achieve this increase in moisture and palatability while retaining the shelf stability against microbiological degradation of a semi-moist food. Accordingly, much research is devoted to this area. There are several known means of achieving some stability at high moisture levels and high water activity. To some extent, it has been possible to increase the moisture content above 50 percent while maintaining the water activity at 0.90 or below by using linear aliphatic 1,3-diols having 4-15 carbon atoms in the chain and derivatives thereof. It is also possible to achieve stability at a high moisture range of about 35 to 60 percent using combinations of glycerol plus either propylene glycol or 1,3-butanediol in a neutral pH range and with potassium sorbate, an antimycotic.
Furthermore, there are two other major disadvantages to the prior art pet food which use a high percentage of propylene glycol and 1, 3-butanediol. Firstly, both of these compounds are petroleum based. Petroleum based additives can be in short supply as evidenced by the most recent fuel crisis and shortage of these petroleum bases. Furthermore, several reports indicate that in the future a severe shortage of petroleum based products will result from a shortage of petroleum itself. Accordingly, it is desirable to minimize the use of petroleum based products. Also, these components in sufficient quantities provide a bitter taste to the food. This bitter taste and the petroleum based characteristics indicate the desirabilities of minimizing the use of these products.
Like diols and glycols, acid used for stabilization has sometimes unexpected and undesirable results. The acid may produce a tart or sour taste in the food thereby interfering with the organoleptic properties of the food. Sometimes, the acid is also capable of emitting an unsuitable odor which even further affects the organoleptic capabilities of the food. Thus, if the use of acid can be minimized, while retaining the stability of the food, desired results are obtained.
However, these methods and similar methods are limited in that there is no satisfactory way to predict the stability of food at these high moisture levels when the water activity exceeds 0.91. The method used now is basically a trial and error method requiring formulation of a food and then testing the food for stability. In addition no scientific way is known for predicting the microbiological stability of foods having such high moisture contents (above about 35 percent).
It is more desirable to replace these methods and similar methods with a more scientific method of predicting stability in products with water activities above 0.91. It can be of great benefit to the food technology art if there is a method provided for predicting the stability effect of various ingredients to form a desired stable food even at the higher moisture levels. This food need not necessarily be clearly definable as either semi-moist or moist. The critical feature of this food is that it have the desired moisture level to provide a desired level of palatability and texture while at the same time retaining the desired characteristics of a shelf stable, semi-moist food. These opposing factors of increased moisture and microbiological stability mitigate against classification of the food in either the semi-moist or the moist category. Even though such features are contradictory, it is nevertheless clearly desirable to achieve both goals.
It is generally known in the art that a semi-moist food usually must incorporate an antimycotic agent to prevent the development of mold growth. However, an antimycotic has many disadvantages. This costly additive can impart a bitter metallic aftertaste to the food, thereby having an adverse effect on palatability. It is therefore necessary to mask the flavor of this ingredient or use a sufficiently low dosage to avoid the problems thus created while maintaining the desired mold free characteristics. It may also be desirable to eliminate this component from the food to avoiding masking and other problems while retaining the mold-free characteristics.
Some attempts are known which eliminate the use of an antimycotic agent. One attempt uses high concentrations of (preferably 20 to 25 percent) aliphatic 1,3-diols having from 4 to 15 carbon atoms. This attempt suffers from two problems. The use of these four carbon atom diols in foods as yet does not have the approval of the Food and Drug Administration. Also, it is not effective in all cases because sometimes antimycotic is still required in certain formulations.
Another method utilizes high levels of propylene glycol in combination with starch modifying material at moisture contents of 20 to 35 percent to produce a product that does not require potassium sorbate. The use of glycerol mono stearate as the starch modifier is required to retain the moisture readily and preserve the texture of the product during storage. The energy required for the high processing temperatures of the method adds greatly to the cost of the food and is therefore undesirable. Also, the problem with adding high levels of farinaceous materials is that it contributes to the staling of the product even in the presence of the starch modifier. A stale product is undesirable and loses the desired soft moist texture. Storage permits the product to stale due to the starch and produces the undesired texture. Thus, a substantial number of problems exist when attempting to reduce the amount of or to remove an antimycotic from a semi-moist pet food and in trying to increase the moisture content of a semi-moist food. It is clearly desirable to solve these problems.
It is highly desirable to be able to predict food stability especially at higher moisture contents. If such a method of predicting stability can be developed, and combined with the production of higher moisture content foods, a food having improved palatability and improved texture due to increased moisture content and shelf stability similar to that provided in a semi-moist food may be achieved.
Thus, it may be seen that there are still many obstacles to achieving both high moisture and shelf stability in a food.